Communication system



Aug. 10, 1943.

R. H. CAMPBELL COMMUNICATION SYSTEM Filed Jan. 16, 1942 s sheetssheei 1 fraz/@20M Ag. 1 0, 1943. R- H CAMPBELL 2,326,617

COMMUNICATION SYSTEM Filed Jan. 16, 1942 3 Sheets-Sheet 2 f '2 A Zig @9 if D] I i Z I 1-70 I `L75 (i6 5727/0/1/-1 67 05 6;@ g v -1 77f1 71 zz 66 f4-7 o si., S)/ L l J7 Aug. 10, 1943.

R. H. CAMPBELL COMMUNICATION SYSTEM Filed Jan. 16, 1942 5 Sheets-Sheet 3 Vdireentre'concimi switcii'is" operatie seicnjqeigtd Y of master stations each provided witiiwaloud-L Y Patented Aug. ie. 1943 f UNITEDI srnr'ss- Richard H. ambel'l; Racine.'` Wise, assign-orY to" Webster Electric Company,'Racine;V}is., av cora poration of Delaware connect the loudspeaker at theinaster stationto. 15 the inputY and output of thearnplifierand tanecusly" to connect theA selected substa 'on loudspeakers to the out'put'andinnut of plie'i' so that comniunication'mayle carriedon between the master and` suo-stations alternately` in opposite directions. A secondt'yp'e of systein comprises a plurality speaker', an arnalier', a station *selector switch or switches and acoinmunicationdirection conto aninteistation circuit 4for reception of call frein' other stations andoperable into a yirstdisfg placed'postio'n in whichtheV interstation lineis connected to the input of the ampliiierand ther` iodspe'aker to the output for reception through the" arr'iplier,v olfierablealso ntoa second displaced position in which the' interstationcire cuitA is connected to the output wandthev loudspeaker to the input of the amplier fortrans'- 35n missionfthrcughjthe ampliiier. Systems of this type may also'include substations provided with' loudspeakers only. Y Atliird type of system also includes a' number ofniaster Vstations each provided, with aloud-4 sneaker, 'an amplifier, a station selectorgswitchor. switclies and a communication direction control.` switch. The communicationl direction control" switch nornfiallyconnects the loudspeaker tiran'z interstation circuit individual to'that stationierthe reception of calls and isl operable' into" a'fdsplaced positionwlierein the loudspeaker isf'c'onnected to the input of the' amplier and the'out'; put o1" the amplifier is connectedto 'a' selectedine terstation circuit individual'to another' station?- A fourthtype of systemgis made up ofa num-y her of'stations each providedfwith a receiver; .af microphone, an amplier to theinputjjof'which'Y the inicrophoneis connected, andstation selec; i tor-switches for connecting the output"of*the' 55 aiilpiin''writer-statici? rf i other stations. j In this type ofs catioi'betwen s siinilarf unixI tern, and

ses?? the .Pf

contempo improvementovri the arrangements. =dSQ10Se Y Other objects Vand advantagescfthe present.. invention' will becon1=- appa-rent from the ensuing description, in` the course', ofA which refere nce` ishadto the acccmpariyingf 'drawings-dn which:

Fig; 2 is a'diagraminatic representation-Oria p'ortifono ther system and illustrates india'rammatic form a untfccnstructed in accordance Witnthepresnt invention;

Fissg is asimilar representation ora portion; of thesysternincluding various formsfof'units hersteforeilsedL-and .l y.

, Eis-1'. 4 iS "2Q/19W .Simi12-11. i9. Eis- .2 .of .a slightly'.y mofiiedrm Qu11ii-99esiructed-inaccordance with the' present invention. 1

Referring :new to'the drawings. the. cpramufn'- cation syst'emgiilustrated therein is-shownft'o con= sistmof sis:- stations; numberedi to; Sinclus'ivey stations;A l andi 2;.'b'eingf showny int Zaritl;41,.'V respectively; and the remaining statins'in Figtf.

Stations i and 2 have units embodying the present invention, and in order to simplify the followinJr description, these two stations will be referred to as esecutive stations. Stations 3 and 4 each include a ,master type of prior art unit and will be referred to hereinafter as sub-executive stations. Station 5 includes aspeaker and a number` of vswitches whereby it may call the executive and certain of the sub-executive stations, and it will `be referred to hereinafter as a subordinate station. Station S,

the executive type and may have aunit corresponding'in construction and arrangement'with the units disclosed and claimed in my co-pending application Serial No, 343,390 iiled July l, 1940. Before considering the details of the circuit arrangement and operation of the system as a whole and of the various units, it may be wellH briefly to describe themain elements located at each of the stations. y

Station II includes an amplifier I0, a microphone I I, a loudspeaker I2, a multi-position communication control switch indicated generally by the reference character I3, and two groups of station selector switches Iii and I5, espectively. The loudspeaker I2 is permanently connected to an interstation circuit i8 individual to station I`for thereception of calls from other executive stations and from sub-executive stations of the type of station 3. In the normal position of the communication vcontrol switch I3, the' position in which` it is indicated, the loudspeaker is also connected to the output of the ampliiier for reception from sub-executive stations ci the type of f The-apparatus at station 2 is very similar to v that at vstation I, as will appear hereinafter. It includes an amplier 25, a microphone 25, a loudspeaker 2'I,f a' communication control switch 23,

and two groups of station selectorV switches A25 and S Loudspeaker I2 is lpermanently con-- nected to the interstation circuit 22 individual to station 2 for the reception of calls from other executive stations and from sub-executive stations of the type of Vstation 3. In the normal position of the communication control switchj, the position in which it is indicated, the loudspeaker is also connected to the output of the amplifier for reception from sub-executive stations of the type of station 4 and from subordinate stations oir the type of station 5. Calls from thetwo latter types of Stations are received over another interstation circuit `2li individual to station 2. Circuit SI is normally connected to the input of the amplifier, as will be described hereinafter. .The output of the amplifier adapted selectively to be connected by station selector switches to interstation circuits I6, I8, I9, 2t, and 2|, individual to stations I, 5, Il, 3, 5, and I, respectively.

The apparatus at station 3 includes an ampliiier 32, la loudspeaker 33 utilizable both as a receiver and as a microphone, and a combined amplifier power Vand communication direction control switch 34. In the normal position of the which has Y not been illustrated in detail, is a station of fier a loudspeaker tt utilizable both as a reoeiver and as af microphone, and a combined ampliiier power and communication direction control switch lli. In the normal position of the switch, the amplifier is rendered inoperative and the loudspeaker is connected to the iiiterstation circuit I individual to station through the switch andthrough a normally closed selector switch 42. munioation control switch, the amplifier is rendered operative and' the'loudspeaker is connected to the output of the amplifier and the selected interstation circuit or circuits are connected to the input of the amplifier, and in a second dis- -placed position, the amplifier is again rendered operative but the connections of the interstation circuit and loudspeaker to the amplifier are reversed. j

'Ihepapparatus at station 5 includes a loudspeaker utilizable both as a receiver and as a microphone, permanently connected to the interstation circuit Irrirndividual to station 5. The station also includes normally open switches d5, i5 and YTI for connecting the loudspeaker to the interstaticn circuits i7, I0Y Si vindividual to stations I, 4 and 2, respectively, whereby station 5 may call the latter.'

The apparatus at station 6, which has not been illustrated in detail, includes'a loudspeaker (not shown) connected to the inteistation circuit 2i individual to station 6 and a number of station 'selector switches (alsonot shown) for connecting the output of the amplifier (not shown) at station 6 to the interstation circuits i5, 20 and 22 individual to stations I, 3 and 2, respectively.

Returning now to Fig. 1 and station I, it may be noted that the amplifier is of the inverse feedback type and comprises three stages of amplification provided by tubes 5I, 52 and 53, The ainplifier comprises also an input transformer 54 and an output transformer 55 having two secondary windings 55 and 5i, the iirst of which is utilized to provide inverse 'feedback and the second of which is used in the usual manner for the transmission of vamplified signals. The winding 56 has one terminal connected to ground and its other terminal connected to the input of tube 52 through a resistor 58, which may have a value of about 10,000 ohms, and through an adjustable connection 55 providing means for adjusting the gain of the amplifier, as hereinafter more fully'described.

The amplifier is suppiied with power from a suitable alternating current source through a transformer 50 and a full wave rectifier I. Power is supplied to the primary of the transformer from any suitable source whenever a switch @2 is closed. This switch may be part of the volume control switch toV be described hereinafter. The

power supply circuitmay be of any well-known In one displaced position of the corn-' munication control switch rather than conjointly by the communication control switch and a high volume switch. However, in order that a better understanding may be had of the operation of the system as a whole, it is deemed desirable briefly to describe the apparatus atstation 2.

. The amplifier 25 -corresponds in construction to the amplier i3 and is provided with anrinput transformer-having a primary winding lll and 'an output transformer 'having two secondary windings lll and H2, the former beingV utilized to provide inverse feedback through. a resistor ||3. The amplifier power supply corresponds identically with that at station and since itis not directly related to the manner in which communication is carried on between stations, it will not be described further.

Tie loudspeaker 21 is permanently connected to interstation circuit 22 individual to station 2 through a volume control indicated by reference character |24, The loudspeaker 21 is connected for reception at `high volume whenever the coinmunication ccntrol'switch 2B is in its normal or middle position and when it is-inits uppermost position. In both of these positions the resistor ||5 is short-circuited by switch blade il@ forming part of the communication control switch.

In the lowermost position of the latter, the switch blade H3 removes the short circuit from about resistor ||5 and connects a resistor lll in paralle] with the voice coil of loudspeaker In the lowermost position of the communication control switch, the inverse feedback is removed from the amplifier as a result of the grounding of the feedback circuit by switch blade l i3 forming part of the communication control switch.

The communication control switch normally occupies a positionconnecting circuit 3| individual to station 2 to the input of the amplier and the loudspeaker 21 to the output of the amplier. Interstation circuit 3| is connected to the input through a normally closed selector switch I |9 individual to station 2, a pair of switch blades |20, and a circuit |2| leading to the primary windingl HG of the amplifier. Loudspeaker 21 is connected to the amplifier output through a pair of switch blades |22 interconnecting circuits |23 and |24, the former of which is Yconnected to circuit 22 and the latter to the secondary winding i2 of the output transformer.

,The conductors of circuit |23 have interposed therein the resistors |25 Yin order to prevent they output transformer winding ||2 from partially short-circuiting the voice coil Vof loudspeaker 21.

The communication control switch 28 is operable to a firstV displaced position to connect the microphone 26 to the input of amplifier 25 and to connect the output of the amplier to the first group of selector switches 29 and into. a second displaced position to connect the output to the second group of selector switches 3. When operated into the rst (lower) of these displaced positions, the microphone is connected to the input of the amplifier through circuit |26, switch blades V|28 in their lowerinost positions, and circuit |2| At the same time the amplifier output is connected to the rst group of selector switches 29 through circuit |23, switch blades |22 in their Vlowermost positions, and a circuit including the branch circuits |21 and |28 theY for connecting the output of the amplier to the cuit' iaindi'viduai tostados In the 'second (upper)" of thetwo 'displaced positions of the communicationcontrol switch, the microphone is connected to the input of the amplier through circuit |26, switch blades |20 in their uppermost positions, and circuit |2|. At this time the output of the amplifier isconnected to the second Ygroup of selector switches vthroughv circuit Y|23 and circuit |33.- This output 4circuit may be selectively connected to one or more of stations I, 3 and 6 through selector switchesY |3|, `|32 and |33, respectively.- f Y Y The'busy signalling arrangement at station 2 includes thefpilot light |35 normally energized Y byA 4a'circuit including the switch` |36 heldin closed positionby la solenoid |31 normally suppliedv with suicient energyto operate and hold the switch |36 in its closed position through a circuit including resistor A|38 leading tothe Bf supply. One terminal of the solenoid is grounded and the other connected by a conductor'l39 to a point between resistors`|4| and |42 connected. in series across circuitrl3. The other terminal .is also connected-to `the communication control switch by a conductor |43. In `the uppermost position ofthel communication control switch thev conductor |43 is connected by switch blade |44 and conductor AE45 to a'point between resistors |46 andV |41V connected in series across circuit plifier 32 having an input transformer with a pri- 34V is `operated into either of itsh'displaced posi-j tions. -In the normal position the f B supply isA cut oil` and the loudspeaker 33 ,connected to the circuit 29,k individual to station 3, through'cir-l cuit 59, `switch blades ISU and `volume control itl. of the amplier vwhenever the communication control switch is operated into either of its displaced positions through circuit |32, and the amplifierf output 'is simultaneously connected by switch blade 863 and circuit '|64to the selector switches 35, 36 and 31. "The busy-signalling arrangement at station 3 includes a light |66 normally supplied with energy through a switch |61 heldin closed position by a solenoid |68 supplied with energy from the B supply through a circuit including resistor |69. One` terminal of the solenoid is grounded and the otherconnected to a point between resistors |10 and l1! connected in series across circuit |34. Therarrangement includes also a pair of resistors |12 and |13 connected in series across circuit 2 and switches |14 operable with the selector switches to ground the point between the resistors through conductor |15.

Theapparatus at station 4 includesthe amplier 39 having an inputtransformer with a primary winding |11 and an output transformer having a secondary winding |18. The amplier is The loudspeaker is connected to the input and switch two stations, the communication from station I tor station 2 extending throughrthe microphone and amplifier at station I'to the loudspeaker at station 2 and in the opposite direction from the microphone and amplifier at station 2 through the Speaker at station I.

`In case the operator at station 2 iinds it dif-V ficult to hear the operator at station I, he can so advise the latter and the'latter may thenoperate the high volume switch 66 in order to increase the gain of the amplifier. If this is done the amplifier gain is increased by grounding the inverse feedback lcircuit by switch 58 and simultaneously decreasing the volume'of. reception of loudspeaker I2. By doing this, the feedback is maintained at the same level to prevent undesired howling. It is' obvious, of course, that, if desired, the apparatus at station 2 could beidentical with'that at station I. In this event' if high vvolume switches at both stationsr were de,- pressed, the 'overall feedback would remain the same because the receiver volumes at both stations would be decreased in the same relation as the amplifier gains were increased.

It may be noted that when station I initially calls station 2, the communication control switch at the latter is in its intermediate position wherein the secondary winding I I2 of the output transformer is also Aconnected across circuit 22.V However, this winding is prevented'from short circuiting the loudspeaker to'any substantial extent by the vresistors 1|25, each of which 'may havea value approximately one-half the impedance of theloudspeaker voice coil andviof the secondary winding amplifier output transformer. For instance, the loudspeaker 21 and secondary winding I I2 may have animpedance of about 50 ohms and the resistors |25 each `a value of about 25 ohms. i Y

The busy signalling arrangement described Vis such that if the operators at either stations 3 or 4 should select either of stations V| o'r'2, Vthen the signal light at the former rwould Vbeextinguished. f

mine event 'station 3 shouid'select stationl v as by closure of selector switch'35, then the solenoid |68 at station 3 is partially short circuitd as it is grounded through resistors |10 and I'II',

circuit |64, selector switch |35, interstationcir-l i* cuit-I6, resistorsfll and |02, conductor'I03. and

switch blade |05 of selector switch 86 at station I.

In the .event station 3 selects station 2, the

solenoid |88 is similarly grounded through the selector switch 36 and the interstation circuit 22 individual to station 2, which is grounded through conductor I4S'and switch blade|52 atistation 2.V

In the event station #selects station'I Vwhile stations I vand 2 are communicating with each Y other, thenthe;v solenoid |94 fatgthe former is grounded through Aa circuit including conductorV |95', switch blade- |99 inits uppermost position, conductor 200, resistors |91 and |98, circuit !BI, selector switch 202. interstation circuit I'I, norrmally'cloedselector switch-ITI atstation I, resistorsV 99 and |00, conductor |38,` switch bladeV the operator at station' 2 performs the functions ergized thereby indicating thatrstation 3 is not.

busy. Theoperator at station ijthen calls station 3, which receives the call through loud-v speaker which is connected to the output of amplifier i@ through communication control switch blades iii, selector switch 83, and interstation circuit 20. To -respondto the call, the operator individual to'station and operatesthe communication control switch to its lowermost posi-v tion. When the selector switch is closed, the solenoids at the two stations are grounded in substantially the same manner as when stations 'i and 2 were placed in communication with each other, i. e., solenoid 92 at station i is grounded through the'switcli blade 'I4 of selector `switch atrstation ii-'and solenoid |38 atstation 3 is grounded through the switch blade |05 of selector switch 83Y at station 1 In order to receive a call, the communication controljswitch 34 at'station .'3 must be inits middle position, and in order toY send a call in the opposite direction, the vcommunication Control switch must be in one or. the other cf'its displaced positions, at Vwhich time the aloudspeaker iszconnected to the input of the amplifier and the output of the amplifier is connected to interstation circuit I3 individual to station by the lowermost blade |63 c-fthe communication control switch and selectoriswitcli 35. The operatorat station need not. operate the communication control switch thereat.

The operator at station imay talk to the operator at station 3 at increased volume simply by closing the highlvolume switch 56. There is no Ydanger of -feedback when stations YI land 32-are communicating with each vkother inasmuch as there is no feedback' path, the "loudspeaker 331 bengalternately used` as a microphone. andreceiver.

WhenV stations i and 3 are communicating with.eachother,` then in case either of stations 2 or i selects one of the twoV stations first mentioned,f`it will receivea Vlousy signal. In the event station 2 selects station v| as by closure of selector switch l3|, then the solenoid |31 is grounded through'. conductor |33, resistors |4| and |2,interstation circuit I3, resistors |0| and |32, conductor |03, and switch blade fc5' asso.- ciated with selector switch E3 at station i. In

the event station 2 selects 'station 3 ,as by closure` of selector switch 32," then solenoid 31 is groundedrthrougha.circuit including conductor |33, resistors Ifaland |42, selector switch |32, vinterst'ation circuit 20, ,resistorsk |12 kanii' |.'|3,` conductor |75 and switch bladeV I'I associated with selector switch 35. j

Station 3 can selectstation i simply by closing selector switch 35." In the event station is not busy, Vthe solenoid |68 remains energizeduntil the'operator at station closes selector switch 88, The operatorl at station 3 thenr callsY station at station 3 closes selector switch 35` hy operating the communication control :switch 34 to either of its displaced positions, Yaiidthe call will be heard by the operator at station through loudspeaker l2..

The mode of operation -betweenstations 2 and 3 issuhstantially the saine as' 'between stationsand It is believed, therefore, that `urti'ier description `thereof is unnecessary..

3. Communication between station el and stations 1 and 2 In the event station desires to call station 4, the communicationcontrol switch is .rst opat Station 1.resisiors-rea'ana' am,V conductor. 98;

switch blades? vin either its middle orlowermost "position, conductor-N4, conductorl', and switch l f l blade HB5 associated with selectorY sv'iritchill.v

rIn the Yevent station B-selects station then station 3` receives #afbusy'signal as the solenoid t68i-is grounded through a circuit includingre` sisters fandil-l, circuit lltselector. switch 3.5, int,erstation circuit` Ifresistors' ll andi/|62.

atstation I, conductor |63; .andswitchblade |65 associated withselect-or switchd.

erated Yintoits uppermostV position. in' order to ascertain Whether or not station switch isopera'ted between its middle lowermost positions, as will lbe described.

To selectstation d, `the cornmunicationcontrol switch isoperated to itsuppermost position, :as already stated, and selector switc closed. It is assumed, iirst, that station 4 is not busy and, therefore, `solenoid remains energized. `r`.Upon

ascertaining that station l `isl.nothi1.sy,'the op? erator at' station 'i operates the communication control switch to its lcwermost position, at which time the microphone Ei is connected to the in# il is although thereafter the4 commun" ation controlk f tween its middle and' lowermost' positions, as-de'-v scribed` above. 1When' stations 'l and 4* are in' v, carriedon in a manneracorresponding tothat dei 7 put ci the amplifierand the output of the vatnpliisconnectedto loudspeaker lo aostation t through switch bladesl-'iofthe communication controlswitch in their lowermost positions, circuits Si 'and 82, selector switch'fi, interstation circuit i9, selector switch 42 atstationil, circuit circuit including interstation circuit "ii-selector switch' til, 'circuit' 32, switch blades l2, and cir- 1 cuit '53.1. Y, .A

It may be well to mention that if the'opefrator at station ,4 desired to do so, he could close'sel lector switch 2%2 inividual to stationl, but'this would have no effect on the circuits exceptrto connect circuits il and EQ in parallel. Y y

In the V'event the operator atjstation desires to talk to station d at high volume,"thehigh` volume switch Vail'station l is operated.

While stations i and Aijare in communication with each other, then stations 2 and 3 receive busy signals if they selec-tgtation and station 2V also receives Va busy signal in case it'selejctsfstaconci@ jj Station 'i obtains a busy signal upon-selecting v:tion-..55

When 'sta-tion y n tion ithen stations and 'receiveliusysignals' Y station il; solenoid-itl ats'tatiorvr 2"is.'g roi1nded througha Circlet-including conductor |39-,v rev-f `sisters 'it-1| and Vlsf-.2; selector switch Lrinter-` When: station dfdesires 'to'call station I|f,`it is necessaryfor theoperator at theformer only to close selector switch 202 individual to'station i. VThis eectsthe connection Yof the loudspeaker V4i) latV station' Zfto the input'of the lam-plier l0' atstationrl through interstation circuit l1 'indi-2 vidual-to station |.l V* Chereaft'e'r, the'direction.of`Vv communication is controlled by fthe operation of communication-control'switch latstation |v becommunication'` 'with each', other;l the other `strations selectihgeither receive/busy; signals,ias

describedalcove.` Y. ,K Y Y :Communication lbetv'veen' stations 2 andfdl is scribed aboveA in connection with stations. 'andfAQ sot-hat `furtherdescription` is deemedyun'necesf Saryi f 4J Communication; between 'station 5 'anniY j stations1 1` and y2, l

. For statioif `te can station 'af-tue opertofat tha-former station closes selector switcjh183. indie'.

vidual to. station 5 and operates the communica'- tion control switchll-to its lowermostposition'.` When lthese operations are eectedmicrophone :iii at station l .is connectedto thefinput off'ai-n- Pier-iil `and the output of .theiamplier is lcon-J nected through Iswitch blades]tthecommunica# tion control switch andv selector. switch.83 yto interstationcircuit |Sindiuidual,to'station and to which loudspeaker 'd4 ispermanently coni-`Vv nected. Communieation in14 the 'opposite direction isleffecte'd simply f'by 'releasing' oozrm'iumicationcqouoiswnch is to retur'nt fijo its middle posi; lis communica-tion 'with stai in the levent theyiselect lciti/1er of stations l-"o'r l5 and .station 3,- r'ec'eives a busysignal in caseit selects station l. V'ln'the event station Zselects 'Is-tation circuitl I6, vresistors |91 and |92 atstation Al., conductorA |9373, and theswit'ch'blade- |55 asso- G, lects stati-on 5, which-it does with its {communi-5 station becauseY the solenoid i3? .thereatfis grounded through a circuit including conductor |39, `resistors iti 'and "M2, s'eieetor switch v |3| individual to station i, ihterstation eircuitf'lii," re-V sistors it! and ISE, conductor igandswitchY blade |65 associated selector-:switch 845. In the event station 2 selects stationffl, Ait re-' f ceivesa busy signal as solenoid i3? `is grounded through a circuit 'including vconductor its, con-'f c ductor |133, switchbladeV liin-itsuopermost po?Y sitioh (in which position a' station of fthe type of station il is selected), conductor |45, resistors and |42, selector switch1,28-irniividualtostaV tion il, interstation circuit le, selector switch 28B f ifInth'eeventstations-selects station twhenfthe latter isfinco'mniunication withstfvalti'ontheirI` solenoidlf at Ystation 3 fis grounded througha" A circuit includingV resistors Hi) *and illfl, .circuit cationcontrolswitch 23 in lits untermostposition'",'`

'solenoid i3? at Station 2 is grounded vthroughfa.v Acircuit jincludhigxcori'ductor liti, conductor |43,

switch blade 'll in `its `uppermost position, con? ductorje, resistors hlt andl-, ls'ele'ctorswitch .i

. individual to l-stauon axiriterstaoon circuit i8, #selector 'switchz iat station i, resistors Vf9.3'

-and lil-ll, conductor'd, switch blade '9? in either i its `'middle or lowermost' positionconductor'sflt and uns, and switch tiene los. associated with;

Station 5.

Y obtain busy signals, also as described above,

|64, selector switch 35, interstation circuit I6, resistors I I and I 02 at station I, conductori03;

andv switch blade associated with selector' switch 83.

In the event station L4 selects station I ,i then blade is when station 4 is communicating with station 5. Y`

In the event station I selects station 5, the soiene-id 92 at station I isV grounded through the solenoid |94 is grounded through a circuit 'Vincluding yconductor |95, switch baseV |99,.conduc'` tor 200, resistors |91 and |98, circuit I8I, selector switch 202, interstation circuit I1, selector switch 1| at station I', resistors V99 and |00, conductor 98, communication control switch 91 in itsl middle 'ductor |95, switch bladeV |99', conductor 209, re-

sistors |91 and |98, circuit I8I, selector switch 203 individual to station 5, interstation circuit I8, selector switch 83 at station I, circuit 82, resistors 99 4and |00, conducto;` 98, switch blade 91 in its middle or lowermost position, conductors |04 and |03, and switch blade I 05 associated with selector switch 83. v n f For the operator at station 5 to call station I, switch 45 at the former-station is closed, thereby to connect the loudspeaker .44 to'interstation circuit I1 individualto station'IVwhich in the nor mal position Yof the communication control switch at station I is connected -to the input of the amplifier.V In responding,.the1 operator at station I preferably closes selector' switch 83 individual to station 5, while the operator at station 5 preferably releases switch 45. Thereafter, the ccm v municationbetween stationsl and 5 is Vcarried on `as described above and the other stations' will lll case they select either stations I or 5.

Communication between stations 2 'and 5 is carried on in the same manner as betweenrstations I and 5, and further description thereof is deemed unnecessary; f f

` "5. Communication between stations 4 and 5 y For station 4to call station 5, the operator at station 4 closes selector switch- 203 individual to station 5. VIf station 5 is not communicatirigwith stations or 2, then solenoid |94 at station 47,.

remains energized to indicate that station 5` is not busy. Thereafter, the operator at station 4 Y loudspeaker at lstation 4 through a circuit ini above described circuit at station to circuit 82,

vvand thence through selector switch 83 individual to station 5, interstation circuit 8, selector switch 293 at station 4, circuit I8I, resistors |91 and |99, conductor 200, and switch blade |99 in its middle or lowermost positions.

In the event station 2 selects either of stations 4 or 5 with itscommunication control switch 2B in its uppermost position, then the solenoid |31 thereat is grounded through circuits similar to those described above in conjunction with the selection of either of these two stations by station I.' In brief, when station 4 is selected by station .'/2 the ground circuit extends `through interstation circuit I9,and when station 5 is selected, the ground circuit extendsv through interstation circuit I8,

In order for station 5 to call station 4, the communication control switch at the latter has to be left in its middle` position, at which time the circuit'ISI is connected to the input of the amplifier and the loudspeaker at station 4 is connected to the output. When station 4 is conditioned for reception of callsfrom station 5 in this manner, then station 5 may call station 4 by closing switch 4S.' Closure of this switch connects'loudspeaker-M to interstation circuit I9 individual to station 4, which circuit is normally connected by selector switch 42 at station 4 to circuit ISI vand consequently to the input of the amplier. j A

In responding to a call, the operator at station 4 preferably closes selector switch203, individual to station 5, and the operator at station 5 releases switchv d6, although this isnot essential, as all that the closureof switch 203 does is to place-interstation circuit I8 in parallel to interstation circuit I9. Thereafter, the operator at station 4 operates the communication control switch thereat between its middle and lowermost i :positions alternately to listen Vand talk to station between station 6 and stations 1, 2 and 3 y, stenen c, it will be remembered, has a ieud- 6. Communications speaker connected to ,circuit 2| individual to sta-A cluding circuit I8I at station 4, selector switch- 'Y 20.3, and interstation circuit I8. Toreverse the direction of communication, thecommunication control switch 4| is released to return it Vto vits middle position, thereby to reverse the'connections of the loudspeakers 49 and 44 to the am` Y,

' signals. In the event station-| selects station 4.,

which it does with4 the communication' control switch in its uppermost position, then solenoid" 9K2 is grounded through` a circuit includingv conductor 94, switch blade 91 in its uppermost position, conductor 98, resistors 99 and |00, circuit 82, selector switch 84, interstation circuit'IS, circuitl I8I, resistors |91 and |98, conductor 200,V

and switch blade |99 in either its middle or lowerrnost position,in which positionsfthe switch cating'with each other.

tion` 6, and a microphone connected to the input of an amplifier, the output of which may be selectively .connected by selector switches to circuits I6, `2i! and 22 individual to stations |,3 and 2, respectively. For station I torcall station 6, the

' operator at station I operates communication control switch I3 to its uppermost position and closesselector switch 81, thereby to connect mi-` crophoiiel II vto the input of amplier I9 and the output of the amplifier to the interstation circuit 2| individ'u'alto,station 6. To respond `to a call. the operator at station` 6 closes the selector switch Atl'iereat to connect the amplifier output to inter- Vstation circuit IB'to which the loudspeaker at station Iis connected. Thereafter, communication ,isY carried onfbetween thestations without any further manipulation of switches, just as in the case when stations I, 2 -and 3 are communi-A Stations 2 and 3 will not receive any busy signal when selecting station 6 when the latter is in Ycommunication with stabinatl'o'ri including,'one or more stations fof `Ia Erst tion circuits individual to other "stations,` n'or circuits frornone or more stationsof thetrst typel `one* of said last mentioned communication cirtion yl l-but-will receive signals uponfselecting-'station il, 'as Will station 4Yuponfvselecting station l, in aimanner.heretofore-described.; 1

--Station kG Ycan call Astationfi l'simply .by closing the selector switch connecting the output fof the amplifier ftlrereat to interst'ationci-rcuift it Upon receipt of-thecall, the operator at-sta'tionl'mcves communication control switch f3 to lits-uppermost positionand -closes selector-switch Sl'Laft'er which communication is carried on Withouturther manpulationofswitches. 1?

The operation when either -of "stations 2 or 1S cal-ls the other corresponds vto `the operation .when station l calls stationair. v l 2f f The procedure when station 3 coils 'station is the same asrwhen station -calls-eitherlof sta*- tions I or 2, except, lof course, the operator vat station EV doesnot have to vmani--pu'date'fany'communication control switch-f none bei-ng provided. For station 6 .to call station 3, itis vnecessary oilily the operatoria-t st-ation to close the' prop-er selector switch.g In responding-the operator at station 3 likewise has-to close the proper selector sev/itch and Vtheir manipulate communicationeo'ntrol'stv-itch 4- i-rcm its middlev to either of Y its displaced posit-ions, as described above; I

While the present invention hasbeen described -connection with avpreferred" embodiment thereof, it should be apparent to those/skilled in the art that therprinciplesare not glin'iitedto 'the `30 sciiic embodiment or to tiiepa-rti'cuilar of various typesjof units-illustrated. l 'What vc'laimas new` and di re Uni-ted States vLettersfatent is: l

il. inanjintercornmunicating w tem; tlrie'coni binaticn including, s; plmality 'of st ionseaich na-vingamiciophonepa 'loudspealer,'v an" ampli;A fier,- ccmmunicaticn control gswitcrr plurality of communication .ciicuits'sbet' tions, jeaclrofY said communication 'circu c stitutin'g ank individual reicepton'ci'rcui particular' station, a ,circuit ate'acli'st'a 1 nectingthe loudspeaker thereat'to the reception circuit individual thereto, aY transmission fc cuit, station selector ,switch means lor-,connecting said' 1;'rausi'r-ussicaV circuit to communication' circuits, individu'ailto other stat sala-nd `m`eansinclidingl circuit associated with s i ccrnmuiicationcontrol' switch means for connecting s 1d microphone to the amplifier and the"anfipliierr output to .said 'nsmi'ssion circuit.l ,v 2. i En itercornmunicating system, the. coin type each'having a microphone@ loudspeaker an ampliiier, i a multi-position. communicationf control switch, a plurality/tof communication circuits' between ,stations leacn` ofisad communication circuits constituting an individual reception cir-1A cuit fora 4particular station, a circuit at station connecting,the'loudspeaker threatto the reception circuit individualtliereto, a"'t`ra1sr1i`sf sion circuit, station selectorswitchme for con necting'said transmission circuit tolcorifnuno;

more' stations' of a second type, corr1muniea'jtion to one or more stations of.' the .secondtypa said` circuits being 'utilizable' foryboth reception and transmission, and means including circuits con- Y trolledL by said communicationcontrol switch for disconnecting said Vmicrophone and 'connecting` cuits to the amplifier input steaks @from sas es @dem L 3. In an intercommunicatiiig xsystem', the combinationiinc'luding, .onefor :more 'stations of aif'rst type veacli having a microphone, .a loudspeaker,

anamplier, andla multi-position corn-mum'ca- "tion controlaswitch '.'l'laving ,ra yrrorrnal ,position and two displaced positions, 'a v,plurality of cornrnunicationf.circuits bettveenrstationa each of said communication 'circuits :constitutingean rlinz-- divdual reception circuit :for al, particular `sta-1;V

tion, 4a :circuit atffle'aic-h station connecting the loudspeaker thereatv` to 'the*[reception` .circuiti-V individual thereto, fa 'transmission circuit, station 'selector switch meansfor connecting 'said trans,i

mission vcircuit tocomm'unioation circuits ind-ir,4

vidual to other sta-tions; one 'or-more's-tations of stations of the second'type, 'saidcrcu-itsiben'g',

uti

Zable iorboth Yreceptiorfi land' transmission,

said micro-phone connecting said; lastrvr connecting said microphone to'w'tlrreampliaier;

input Vin both displaced positjlcnsggand for con-- nectingsaidtransmissioncircuitfto the afm-pli'erl output in one -o-'said displaced pos ions' and said last mentioned communication vcircuit 'to Vthe amplifier output in the-other displaced position of said switch. fj l, z

4. In an intercomI-nuxiicati-ng system, the com--A lamination incl-udine; D@ Inoregstationsof a rst typeeacli having-,a microphone; --a loudspeaker; an arnplier, and a :multiposi-tion communication rcontrol switch, `a,plL-i'rality of 'communication circuits 'between stations, each constituting an,

individualureception cir uit/for a Ypartcular staf tiene, oircuitat each-Istat.

Ssael' there@ to the 'recept n; 'Gironi n ual thereto, one or more stations of a seco-nd:typev having an amplielr, la' loudspeakerk ,utilizable Ab otlo sie microphone @darete-Wer: recepiiO-nirciiits individual to stationsof thesecondftype extendg type'. having means incll'iding,- aY communication control switch. normallv comaecting tljieloud- Y speaker to its reception circuit-. andoperable.toV

connect the. loudspeaker to ,tne ano erinput andthe amplifier output 4recept on Ycircuits individual to' said kstations ofthefnrsttypeand' said static-ns fof tbe irsttvpewliaving a trans-f, mission circuit, station'selector switchv means for connecting saidtransnisfsion circuitto 4therrecepf tion circuits individual toboth typ 510i sta ons,

connectin'grftlie 'reception circuits. dual, thereto to,said*loudspeakereadfloperableato al r positionfto coniv ct said microphone' to ,th A ne; input `and the-amplineifoutputtoi sind aan 'normally Lconnectin Genneciinefihe loujdj-V i 'ndividf Y I and operable into vtwo .displaced positions wherein said microphone is connected toitheampliner inputand in one of which the amplifier outputA is. connected to lsaidnrst circuitand in the other of which the output is connected'to said trans! mission circuit; .one or more stations of a type havinga circuit utilizable for both reception and transmission, a loudspeaker utilizable both as a microphone rand a receiver, an amplifier and means including a communication Vcontrol switch and circuits controlled r,thereby normally connecting tsaid loudspeaker to'said circuit and oi,- erable to connect said' loudspeaker. tothe ampli-A fier output or input and. simultaneously to conneet" the rampliier input or output tov said ou cuit; `and one or more stations vof a third type each having a reception circuit, a transmission circuit, an amplifier, a loudspeaker utilizable both as a' microphonek and a receiverand means includingfacommunication control switch and circuits controlled thereby normally connecting saidfloudspeaker to saidfrecepticn circuit and operable to connect said loudspeaker to the ain- `pliiierlrinputfand'therampli er output to said transmission circuit; and 4-interstation ci' nits between the stations 'including-circuits individual toi each station to which the loudspeakers are normally connected. f

6.' In anintercommunicating system, the coinbination including, one or more lsteutions of a iirst type each having a microphone, a loud vrspeaker, anamplicr normally operable at predetermined gain and communication control Y switch means, one or more stationso a second n type, communication circuits between stations oi the two types utilizable for broth reception and transmissien,v and means at Astations of the nrstv type including said `communication control switch means and circuits controlledtherebyfor connecting said microphone tothe amplifier input Yand the amplier output to an intcrst'ation circuit and simultaneously increasing the gain ci' said amplifier.

7. kIn an intercommunicatingsystem, the-ccm binationincluding', one ror more stations o first type each having a-microph'ona-a ion speaken'anamplifier operable at high low gain, a` multi-position communication con rol switch having a norinal'- position and ano position, one'or morestations or a secondtype,VV

communication circuitsfromone or 'more stagtions of the iirst type to one yor more stationsof i,

the second type, said circuits being gutilizal mal position to condition saidamplier for 'oper'- ation at low gain; to v discorn'iectsaid microphone and tofconne'ct one of said last `mentionedA com- Y Y inunication circuits tothe amplier input and the amplifier "output to saidloudspeaker, and operable'in said other position to condition said amplier for voperation at high gain and toY conneetA saidz microphone to the ,amplifier inputand for both reception and transmission, and rrneansinf' cluding said communication control switch and* circuitsv controlled thereby operable in said forjaparticular station, a circuit at each station connecting the loudspeakerv'thereat to the reception circuit'individual thereto, a transmission cirlcuit, station selector switch means for connect- 'ing said transmission circuitrto communication circuits individual to other stations, one or more stations of a second type, communication circuits from one or more ,stations'oi the first type to one or more stations of vthe second type, saidcircuits being kutilizable ior Vboth 'eception andrtransmission, and means at each station ofthe rst type including psaid" communication control switch andcircuits controlled thereby for conditioning said amplifier for operation at low gain, for disconnecting said microphone and for connecting said last mentioned communication circuit to the amplier'inputand the amplier output to saidv loudspeaker in the normal position of saidswitch, for connecting said microphone to the'ampliiier input'in both displaced positions, and or connecting said transmission circuit to the amplier output in one of said displaced positions and for connecting said last mentioned communication circuit to the ampliiier output and conditioning said amplifier for operation at high gainin the other displaced position of said switch.` t

.9. InanV intcr'comrnunicating system, the com-` bination including, `one ormore stations of a iirst v typeg'each having a microphonafa loudspeaker normally conditioned for receptioniat high Volurne, an ampliernorrnally' operable at low gain and operable also -athigh gain, a manually operable two position swi'tehassociated with said loudspeakerand amplier forsimultaneously decreasingthe loudspeaker reception Volume and increasing the amplier gain, and a multi-position communication control switch having a normal position anditwo displaced positions,y a plurality of communication circuits between stations, each of` Said communication circuits constituting an individual reception'circuit for a particular station, a circuit at each station connecting the loudspeaker thereat to the reception circuit individual thereto, a transmission circuit, station selector switch means for connectingsaid transyin issioncircuit to communication circuits individual to other'stations, one or more stations ora second type, communication circuits from v one orY more stationsof the rst type to one or morev stations of the second type,. saidcircuits being utilizable for both reception andtransmission, and meansat each -station'of the' first type including said communication control switch and circuits controlled 'thereby for conditioning said amplifier for' operationatlowv gain, forl disconloudspeaker in the normal position oi saidswitch, forconnecting said microphone tor/the amplifier inputy in both displaced positions, andvv for connecting :said transmissionl circuitto the amplier output inioneo saiddisplaced positionsand for connectinA said last mentioned communication circuit to the ,amplier output and conditioning "said amplie'rforoperation at high gain in the Y otherdisplaced position of said switch.

' 1G. In `an interoornmunicating system, the coinbination including, oneV orfmore stations Aof a iirst type eaoli having ainicrophone`,a loudspeak- Ysaid switch.

1l. In an intercomniunicatingsystem., the corn "associ-*r i second type,- said .circuits seing zutnfzabie'toraoth tions, .a plurality of communication -circuits tween stations, each of saidcommunication circuits .constituting an individual recenvation circuit .for apartiwlarstation, a circuit at each-station tion volume,` for disconnecting said microphone and for connecting said last mentioned communication circuitto the amplifier input and the ani.-

pliiier-.output to said loudspeaker .in the norm-al positionoi said s-wi-tcl1,fiorconnectingsaid micro.-A phone to the amplifier input inhoth displacedA positions, 'for connecting saidtransmission circuit tothe amplier outputiin che o said. displaced positions and for comiecting said lastinentioned communication circuit to the ampliiier.outr put and conditicuingsaid loudspeaker at low reception volume and said ampli-fier for operation at high gain inthe Vother displaced position ci bination including.l arf plurality of' stations. each having a'micropnone, a loudspeaker, an-ampli1-."

fier, communication conttclsWitch-means, a plus rality-of communication circuits betweenstations, each of said communication circuits constituting an individual receptionv circuitiora particular station, a circuit at each station connectingthe loudspeaker thereat tOi-the reception-circuitin-Y vdividualthereto, a transmission circuit` station selector switch meanstorv connecting said. trans-l Y,

mission circuit to.' communication circuits individual to otherv stations, grounding switclies'assoa ciated Witnsaid. .selector switchfmcans,..s ignal means, *signal-` controlling. mea-ns, circuitfcom necting said controlling mea/ns. tosaid'itransmis-` sion circuit, and means including'circuitsasso ciated' with said .communicationcontrol switchA means i or connectingsaidmicrophone toItlielaino-` ,plieir input and theamnli'er outnutftofsa-id transmission circuit andjor connectingsaid-sigf nal controllinrI means to qsaid grounding switch. means. L j f L 1 12.ln. int erccinnmnicating system, the. cornreception andl transmission, and .one -being find# .visual .roesch station infin@ nrs-t type anal-the' Y,

others individual to stations loiftnesecondftype, station .selector switch meansat stationsof the first .type .for connecting the`v two '-V-lway communication circuit itl-iereat'ltoathecircuit's individ'ual y to Vstations :of .the second tyipe, groundingjswitches associated with @both-said station selector switchr means,v a circuit `.connecting .said grounding switch )means to SadFreceptiQn-cirCUJt, and means' at each station. .of-tice rst typebi-ncludinggsaid communication leoni-,m1 switchl'andcircuitsfconL- trollecl.therehy so .constructedandarrangcd that Vin a normal position. ofthe switch said mici-cm.

` phono is;disconr'ieotcd.and said loudspeaker: is

conneotedlto. saidreception circuiti andto the asnplieri output andthe amplrfinpl 1S 011?- nected. :togsaid'itwoway 'communication circuit,

andwherein saidsignal controlling .means .ils dis connected said glnoundi-ng switch means and theilatteraare connctedto saictrcception .circuit and tWo-it/ay.V Vccmm'unication circuit said microphone is connected to the ampiiiierinput Y signal. controlling v t`o""s`a-id bination including, oneor more stati-ons of.r l Y firstA type each havin'go microphone, a loud?.

speaker, an amplier, and. a:mu-lti.-;position com munication control. switch having a normal posi-.1` 4 tion and two `displaced positions: 1a plurality of:v

in both displaced positions;andthat 'said'trar-is'- mission circuit is Leonnoctedto tho-ampier Aodittwo-.way .comnunication circuit in one 'of .said displaced nositonsgfand thatin said other-dis'- plaCQdposi.-tio`r1said tWo-Way-:communication ycirT @uit .isiconnectadf to. me examiner-.outputand said 4Isig-nal .cnntroliingmeans' and" 4grounding `A v switches .'are.; connected as the normalf posi..

tionot'said'switchz-- 1 Y .-'1 '13.. .AnA 'intercom-municating sys-tern l in-couiloination,. i cornici-1e station .of airs't typsieach having( amplifier, ia -m'icrrnalin-rie,l a loudspeaker, .a first. circuiti-utilizable? both for reception? Farid. transmission.; reception circuit; ,a,

transmission circuit, andlrn'eans includingia communication: controlfswiltchqand circuits controlled thereby-for normally connecting said.V loudslceakca1A to both saioznrst .and1recepnon-circcitsaccapari into itwosdisplaced 'positions .wheroin saidv microphone.'A is .connected to-the amplifier inputv heampliiier.-v output icoon: z 1 nested?` to-.said rst.- circuit 'and the otherof and in one of Whicht the" ouput'z isfconn'ccted tov saiditransmis siongcircuit ','z one .morcstations oatyjpehav- Sa;..'cirouit utilizalolerfor both, reception! and transmission i microphone andiafreceiyen, an' ainplierand i meansiincludingv. a communication control:switch and' .'c-ircuitsc-tco'fitrolled. therebynnrmaiiy con# neet-ing said Loudspeaker tosaid circuit andop crableito; Qonnactsaid loudspeaker to. the annali-T crfxontnut orfinn1u1. and..simultaneously itolcon- 'Vnectatlierampliarinputor: outnntsto said circuit;

und; more2 stations oifathirrtitype cach ha ing'gacprecentionrcircuit, a transmission. l'cir' l f cuits armpiel; afloudspeaker utilizable-.both

communication ciscuitsbetween stations, each ofl said Y:communicatien .circuits .constituting 1 ank indiyidual reception circuit. fo-isa' particular station,

a circuit :at'-ea ch 'station conneotingthedoudas; o micronhoneiand 'a' neceiverand; llimsi-,135 fins Cluding-acommunication control andcic Suits;ontrolledtherehyfnormaliyconnectinglsaid speaker thereat to the reception circuit.indi'tlidual:` L thereto, a transmission circuit; [stationv selector means, a circuit permanently connecting said Y sig-nal con-trolling rneansto'said transmission ci-rl' cuit, one or more stations of -a secon-doorzo-V inunication circuits from oneorjimrore; `tions ofthe first ty-peto one orb-more-'staftionsfof=-the l i Asti/Fitch means. for connecting' said transmission lcircuit to communication; circuits individual tof other stations, signal. means.,.signa1i controlling lousisneake said .raccptionliiraitfand optical@ including whereby when a station calls a station communieating with another, the signal controlling-means at the calling station is grounded to indicate the so called station is busy. Y

lli. A unit for use in-,an intercommunicating system, including in combinationa microphone,

, aloudspeaker, an amplier,` communication con- :trol switchmeans, ka reception circuit connected tothe loudspeaker, a transmission circuit,v and means including circuits controlled by said cornmunication control switch means for connecting saidrmicrophone to the amplier input and the amplifier outputqto said transmission circuit;

15.1,Aunitgfor use in an intercommunicating system,.including in combination, a microphone, a loudspeaker, an amplifier, and a multi-'position communication control switchQa reception circuit connect'edto'said loudspeaker, atransmission circuit, a circuit utilizable for both reception and transmission, and means includingocircuits controlled by said communication control switch for disconnectingV saidimicrophone from the amplier input and for connecting'said last mentionedcircuittothe amplifier input andthe amplifier output'to said loudspeaker.`v

16A unitV for use in an intercommunicating system, including in combination, a microphone, a g loudspeaker, ran ampliiier, ar 'multi-'position communication control switch having a normal vposition and two displaced positions; acreception circuit'connected-to said loudspeaker,A a transmission circuit, a circuitjutilizalzlle for both, reception and l transmission, and' means including said communication control switchvv and circuits controlled thereby for disconnectingl said microphone from the amplifier input andior 'connecting said last mentionedrcircuitjto theamplier input and the amplifier Voutput to said 'loudspeaker in the normal position of said switch, for connectingsaid microphone Ato the amplifier input in both displacedpositions, and for connectingV saidI transmission A circuit Ato thev amplier output in one-ofsad displaced positionaand for connecting said last mentioned communication circuit vtothe amplier output-in the, other Y di'sl placed positionlof said switch. V-

17. -A unit for use in ang-intercommunicating system, including in combination, al microphone, a loudspeaker, an l amplifier Yhavinggain control means and normally operablecat a'predetermined gain, communication control switch means, a communication circuitv utilizable: for bothY reception and transmissiom'andmeans including'said communication control switch-means and circuits controlled thereby "for connectingV said microphone to the amplier input and the ampliiier output to said communicationicircuit and'sim'ul- 4 i taneously increasing-the gain of saidramplier. 'r

j 18. Azunit vfor use inan' intercommunicating system, including in combination, a microphone,`

a. loudspeaker, :anv lamplifier operable-at high and lowv gain, aI multi-position'y communication control r,switch having a'fnorrlnal*positionv and anotherposition', a communication circuit utiliz-V able i for both reception and Y li'transmiss'ion, vand c means'i-includin'gsaid communication rVc'fjoiritrol switch and circuits controlled thereby operable in :saidnormal -positionfto' condition said aIIlDli-V lier yfor operation Aat x'gain,VV to Vdisconnect vsaid microphone and rto connectsaid communication arcuate me Yamplifier input ,and the anpuner output to said loudspeakenand operable insaid system, including in combination, -a microphone, al loudspeaker,V anamplier operable at high and, lowgain, agmulti-position communicationY control4 switch having` a normal position and two displaced positions, a reception circuit connected to saidloudspeakerI a transmission circuit, a circuit utilizable for both reception -and transmission, andmeans including said Ycommunication control'switch and circuits controlled thereby for conditioning said amplier for operation at low gain, for disconnecting said'microphone and for connecting said lastj mentioned circuit to the amplifier input and the` amplifier output to said loudspeaker,V in the normalposition of vsaid switch, for connecting said microphone Vto the amplifier input in both displaced positions, and for connecting said transmission circuit to the amplifier output inone of said displaced positions and forv connecting said last mentioned circuit to theY amplifier output and conditioning said Vamplifier for operation at high gain in-the other displaced position of said switch.

20L-.A unit for usein aninterc'ommunicating system, including in combinations,r microphone, a loudspeaker normally conditioned for reception at high volume, an amplifier normally operable at high gain and operable also at lowgain, a manually operable two position switch associated withY said loudspeakerand amplifier for simultaneouslydecreasing the loudspeaker Areception volume and increasing the amplifier gain, va multi-position communication controlswitch having a normal position :and'two displaced positions, `a reH ception circuit connected 'to saidloudspeaker, a transmission circuit, a circuit utilizable for both receptionrand-transmission, and means including said communicationcontrol switch and`circuits controlled thereby for conditioning said amplifierV forV operation at low gain, for disconnectingsaid microphone and for connecting said last mentioned. circuit to the ampliiieiyinput and-the microphonefto the amplifier `input in; both, dis- Y. placed positions, and forconnecting said transmission circuit yto the ampliiier outputinione of c said displaced positions'` and ior'ccnnecting said last mentioned :ircuit to the amplifier output and conditioning said ampliiierV for operation at` high gain Vinthe other switch. c

'21, VA unit or use in an intercommunicating system, including, in combination, microphone,

Va loudspeaker operable at high'and low-.recepl tionpvolume, an amplier operable at high and low gain, a multi-position communication control switch having a normal position and two displaced positions, a-reception circuit connected :to

said loudspeaker, a' transmission, circuit, .a circuit utilizable or bothreception.andtransmission; and means ateachstationof the first type includngsaid communication control switch and 'circuits controlled-thereby for-conditioning said 2lII1Dliiie1-;-:ior; operation atlow gain andsaid loudspeaker at high-,reception Volume, for disconnectingsfaid` microphone `and for connecting said last mentioned communication circuit to the amplier input',v and-the amplifier ,output togsaid loudspeaker in the-normal position of said switch, for connecting said-microphone` to the amplifier 'input i'ngbcth, displacedv positions,V and Yfor connecting'said transmission kcircuit to the amdisplaced position of ,i said t pliiier output in one of said displaced positions and for connecting said last mentioned communication circuit to the amplier output and conf ditioning said loudspeaker for reception at low reception volume and said amplifier for operation at high gain in the otherdisplaced position of said switch. Y

22. A unit for use in an intercommimicating system, including in combination, a microphone, a loudspeaker, an amplier, communication control switch means, a'reception circuit, a circuit at each station connecting the loudspeaker thereat to the reception circuit individual thereto, a transmission circuit, station selector switch means for connectingsaid transmission circuit to circuits adapted to be connected to other units,

system, including in combination, a microphone,

a loudspeaker, an amplier, a multi-position communication control switch having a normal position and two displaced positions, a reception circuit connected to said loudspeaker, a transmission circuit, station selector switch means for connecting said transmission circuit to communal controlling means to said transmission circuit, a conmmunication utilizable for both reception andl transmission, station selector switch means for connecting the two-Way communica-` 'tion circuit to communication circuits, grounding switches associated with both ysaidy stations-selector switch means, a circuit connecting said: grounding switch means to said reception circuit, and means including said communication control switch and circuits controlled thereby to constructed and arranged that in a normall position of the switch said microphone is disconnected and said loudspeaker is connected to said reception circuit and to the amplier output and the ainv plifier input is connected to .saidtwo-way communication circuit, and whereinsaid signal con-Y. l

trolling 'means is disconnected from said grounding switch means and the latter are connected to said reception circuit and two-way communication circuit; that said microphone is connectedv to the amplier input in both displacedposi-V tions; and that said transmission circuit is oonnected to the amplier output and said signal controlling means to said two-way'communica-- tion circuit in one of said displaced positions;Y and' that in said other displaced position saidtwowayv communication circuit is connected to` the amplier output and said signall controlling means and grounding switches are connected as in the normal positionlof saidswitch.

YRICHARD H. CAMmanija..l

4nication circuits, signal means, signal controllimg4 means, a circuit permanently connecting said sig- 

